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They were thought to be sedentary during their solitary phase, whereas they can travel several kilometres. Populations were considered fragile, whereas they are actually perfectly suited to their environment and much larger than previously thought. Desert locusts had us all fooled. A team from CIRAD recently demonstrated this through a wide-ranging genetic study in collaboration with numerous African partners.
The desert locust (Schistocerca gregaria ) primarily lives in the Sahara. When solitary, it is inoffensive. Once gregarious, it is a serious threat to crops. Swarms can total several billion individuals and contaminated up to 30 million square kilometres, from Mauritania to India, and North Africa to Kenya. The damage caused is substantial.
A fair amount is known about gregarious populations during invasions. But until recently, solitary populations, during remission periods, were largely a mystery.
Such populations, which migrate only at night, often go unnoticed in the immensity of the Sahara. Until recently, they were thought to be sedentary and under threat from droughts in this environment with very little rainfall. However, under the right ecological conditions, it is from these populations that invasions develop.
For several years, locust specialists from CIRAD tried to solve the mystery of these solitary locust populations. But how could they identify them, when densities were low and spread over some 15 million square kilometres, in a largely inaccessible region with an arid climate, and how could samples be taken in sufficient numbers to analyse their structure?
From 2008 to 2012, CIRAD scientists organized a huge operation to collect populations during remission periods, in collaboration with all the locust control centres in West Africa, Northwest Africa, Sudan and Pakistan, and the support of the FAO. They were able to collect 23 populations and a total of 580 individuals.
This enabled them to begin a study of the genetic structure of the populations. Twenty-four DNA microsatellite markers were used to characterize the populations and examine their links. This information served to provide a clearer understanding of population dynamics and made it possible to determine their size during remission periods compared to during invasion periods.
The results, which were published recently, were both surprising and novel. It was previously thought that these solitary locusts only existed in the form of small populations liable to extinction and recolonization as a result of the arid conditions in the Sahara.
The CIRAD team's results turned this belief on its head. Contrary to what might have been expected, the solitary locust populations almost all showed high genetic diversity.
Moreover, they did not show any genetic structure, from Mauritania to Pakistan. This means that population levels do not fall sufficiently at the end of invasions to affect their genetic diversity and homogeneity. In fact, populations are much larger during remission periods than previously thought.
Desert locusts seem to be wholly suited to their Saharan environment. While localized demographic hiccups occur, they are rapidly compensated for by arrivals from outside, thanks to the high mobility of the species.
This type of hiccup was observed in three of the populations studied: they suffered high mortality, probably linked to dry conditions, but benefited from recolonization by winged individuals from another population. Populations are thus regularly mixed across the whole of their remission area.
These results radically alter our vision of solitary desert locust populations. This is a boost to invasion surveillance and prevention strategies. It has now been proved that solitary populations, which travel fast, can very quickly discover zones in which the conditions are almost ripe for reproduction.
During remission periods, those zones must therefore be identified as soon as possible, so as to act immediately in the event of concentrations and prevent invasions. CIRAD's locust specialists have recently made significant breakthroughs in this field too.